To protect passengers in a vehicle, such as an automobile, against injury in the event of a collision, the potentially injurious effects of the high deceleration forces acting on the vehicle as a result of the collision must be moderated. One commonly practiced method of moderating the effects is to hold the passengers securely against portions of the vehicle, such as its seats. The passengers thus decelerate at essentially the same instant and the same rate as the vehicle and are not "thrown," as the result of differing times or rates of deceleration, against parts of the vehicle interior.
The most widely used device for securely holding a passenger in an automobile is a safety belt that straps the passenger firmly to his seat. Safety belts may be categorized into two general classes: two-point safety belts and three-point safety belts. A two-point safety belt includes a single belt or strap that is attached to the automobile frame or chassis at two points, located one on either side of an automobile seat. The belt is generally arranged to extend across a passenger's lap, but may instead extend diagonally across his body from one shoulder to the opposite hip. A three-point safety belt includes two belts or straps that are joined together so that only three connections are necessary to secure the ends of the belts to the automobile. Generally, a three-point belt is a combination lap belt and diagonal shoulder belt.
In order for a safety belt to be effective, it must be properly adjusted and fastened around a passenger sitting in an associated automobile seat. The typical safety belt is therefore provided with an adjustably positioned, releasable buckle so that the passenger may fasten and adjust the belt about his body after he is seated. Nonetheless, many people regard having to fasten and adjust a safety belt as an annoying inconvenience, and thus the effectiveness of a safety belt is often wasted because passengers do not use it. To eliminate the inconvenience of having to fasten a safety belt, it has previously been proposed to fasten one end of the belt on the adjacent automobile door. When the door is opened, the safety belt swings away from its associated seat to allow a passenger to enter or leave the automobile without handling the belt. When the door is closed, the safety belt swings into position about its associated seat and the body of a passenger who may be seated in it. The additional length of safety belt necessary for the arrangement is taken up on an automatic winding spool when the door is closed. The automatic take-up also assures a close engagement of the belt with a passenger's body. A locking mechanism on the spool prevents the belt from unwinding when an impact load is applied to the belt as a result of a collision. Such a safety belt arrangement which does not require the user to buckle or unbuckle the belt is commonly designated a passive safety device.
Although the safety belt arrangements described above have been shown to be of value in preventing injuries to automobile passengers involved in an accident, they also have certain disadvantages. For example, a three-point safety belt effectively restrains both the upper and lower portions of a passenger's body, or, more specifically, his torso, but the two part construction and three-point mounting of the belt make entering and leaving an automobile rather difficult, particularly for passengers in the rear seat of a two-door automobile. Moreover, if a three-point belt is adapted to function as a passive safety device, as described above, the cost of the belt, which is already greater than the cost of a two-point belt, is further increased. On the other hand, a two-point safety belt, while less expensive and cumbersome than a three-point belt, can only restrain either the upper or the lower portion of a passenger's body. In the event of an accident, therefore, the unrestrained portion of the passenger's body is free to move and consequently is more likely to be injured.